PuTTY fails to do proper bounds checking on SSH2_MSG_DEBUG packets. The
"stringlen" parameter value is incorrectly checked due to signedness
issues. Note that this vulnerability is similar to the one described in
GLSA 200408-04 but not the same.

Impact

When PuTTY connects to a server using the SSH2 protocol, an attacker
may be able to send specially crafted packets to the client, resulting
in the execution of arbitrary code with the permissions of the user
running PuTTY. Note that this is possible during the authentication
process but before host key verification.